use-cases/codebase-onboarding.md +86 −0 added
1# Understand large codebases | Codex use cases
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3[← All use cases](https://developers.openai.com/codex/use-cases)
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5Copy page [Export as PDF](https://developers.openai.com/codex/use-cases/codebase-onboarding/?export=pdf)
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7Use Codex to map unfamiliar codebases, explain different modules and data flow, and point you to the next files worth reading before you edit.
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9Easy
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115m
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13Related links
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15[Codex app](https://developers.openai.com/codex/app)
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17## Best for
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19 - New engineers onboarding to a new repo or service
20 - Anyone trying to understand how a feature works before changing it
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22## Starter prompt
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24Explain how the request flows through <name of the system area> in the codebase.
25 Include:
26 - which modules own what
27 - where data is validated
28 - the top gotchas to watch for before making changes
29 End with the files I should read next.
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31Explain how the request flows through <name of the system area> in the codebase.
32 Include:
33 - which modules own what
34 - where data is validated
35 - the top gotchas to watch for before making changes
36 End with the files I should read next.
37
38## Introduction
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40When you are new to a repo or dropped into an unfamiliar feature, Codex can help you get oriented before you start changing code. The goal is not just to get a high-level summary, but to map the request flow, understand which modules own what, and identify the next files worth reading.
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42## How to use
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44If you're new to a project, you can simply start by asking Codex to explain the whole codebase:
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46Explain this repo to me
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48If you need to contribute a new feature to an existing codebase, you can ask codex to explain a specific system area. The better you scope the request, the more concrete the explanation will be:
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501. Give Codex the relevant files, directories, or feature area you are trying to understand.
512. Ask it to trace the request flow and explain which modules own the business logic, transport, persistence, or UI.
523. Ask where validation, side effects, or state transitions happen before you edit anything.
534. End by asking which files you should read next and what the risky spots are.
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55A useful onboarding answer should leave you with a concrete map, not just a list of filenames. By the end, Codex should have explained the main flow, highlighted the risky parts, and pointed you to the next files or checks that matter before you start editing.
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57## Questions to ask next
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59Once Codex gives you a first pass, keep going until the explanation is specific enough that you would trust yourself to make the first edit. Good follow-up questions usually force it to call out assumptions, hidden dependencies, and the checks that matter after a change.
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61- Which module owns the actual business logic versus the transport or UI layer?
62- Where does validation happen, and what assumptions are enforced there?
63- What related files or background jobs are easy to miss if I change this flow?
64- Which tests or checks should I run after editing this area?
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66## Related use cases
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68[
69
70### Iterate on difficult problems
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72Give Codex an evaluation system, such as scripts and reviewable artifacts, so it can keep...
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74Engineering Analysis](https://developers.openai.com/codex/use-cases/iterate-on-difficult-problems)[
75
76### Create browser-based games
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78Use Codex to turn a game brief into first a well-defined plan, and then a real browser-based...
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80Engineering Code](https://developers.openai.com/codex/use-cases/browser-games)[
81
82### Learn a new concept
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84Use Codex to study material such as research papers or courses, split the reading across...
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86Knowledge Work Data](https://developers.openai.com/codex/use-cases/learn-a-new-concept)